Technical Field
[0001] This invention relates to an improved energy absorbing rubber composition that is
useful as a structural material in a wide variety of articles. It is especially useful
in shoe parts due to its light weight, its excellent ability to absorb energy, and
its hardness which is in the range of about 20 to about 30. More particularly this
improved rubber composition has a low specific gravity and unique load deflection
characteristic. The shoe products made from this composition are comfortable and provide
the wearer with protection from bonejaring shock which is the main cause of certain
injuries, such as Achilles tendonitis and shin-splints.
Background of the Invention
[0002] Elastomeric compositions which have been blown to give a microporous composition
have been widely used in numerous products. For example, such elastomeric compositions
are commonly used in shoe products, inner soles, sole insert pads, and heel pads which
are preferably of the round cookie shape. Although these elastomeric products have
been widely used in shoe inner soles and inner sole components , it has been appreciated
that they needed to be improved in their shock absorbing ability. In recent years,
a special polyurethane product has been promoted and sold which has a good ability
to absorb shock and which has deflection characteristics similar to that of human
flesh. This polyurethane product is not only extremely costly, but has a very high
specific gravity, usually about 1.4, which results in shoes made from it being very
heavy which is undesirable especially for runners and joggers.
Summary of the Invention
[0003] The elastomeric compositions of this invention have a very desirable combination
of properties for numerous applications as a structural material. These elastomeric
compositions have the ability to absorb large amounts of kinetic energy and thus can
be employed in a shock absorbing capacity. They have an ability to deform in response
to impact and therefore upon contact with the human body they feel relatively comfortable.
The elastomeric compositions of this invention are also nontoxic which permits them
to be used in close proximity to human and animal bodies. This invention specifically
discloses an elastomeric composition which comprises: (1) a polynorbornene rubber,
(2) a plasticizer, and (3) a resin which is incompatible with the polynorbornene rubber.
[0004] Such elastomeric compositions are very valuable as a structural material for use
in many articles of manufacture, such as, shoe soles, shoe inner soles, shoe heel
pads, shoe sole insert pads, gun butt pads, horse shoe pads, baseball glove insert
pads, tumbling mats, flack jacket liners, bulletproof vest liners, helmet liners,
hammer handle grips, carpet underlay pads, floor mats, antivibration mats for sensitive
instruments, and the-like. Thus, this invention further reveals a nontoxic rubber
article having excellent energy absorbing properties comprising: (1) a polynorbornene
rubber, (2) a plasticizer, and (3) a resin which is incompatible with the polynorbornene
rubber. Such nontoxic rubber articles normally contain from 20 to 400 parts of the
plasticizer and from 5 to 120 parts of the resin which is incompatible with the polynorbornene
rubber per 100 parts of the polynorbornene rubber (all parts are by weight). Generally,
these rubber articles are cured with sulfur or peroxides at an elevated temperature
in the presence of a cure activator.
Detailed Description of the Invention
[0005] We have discovered that a polynorbornene based (non-polyurethane) elastomeric composition
can be produced that has excellent energy absorbing properties, that feels good to
touch, and that has a specific gravity of about 0.7 to about 1.2 (preferably a specific
gravity of 1.0 to 1.15). Such polynorbornene based elastomeric compositions can be
produced at an appreciably lower cost than can similar polyurethane compositions.
[0006] The polynorbornene based elastomeric compositions of this invention can be readily
shaped, preferably by molding, for instance, by injection molding, compression molding,
or transfer molding to form shock absorbing articles, such as, shoe inner sole pads,
shoe heel pads and other shoe components. These elastomeric compositions are on a
part by weight basis a blend of 100 parts of polynorbornene rubber, 20 to 400 parts
of a plasticizer and from 5 to 120 parts of an incompatible resin (a resin which is
incompatible with polynorbornene rubber). Such elastomeric compositions also can contain
from 0 to about 100 phr (parts per hundred parts of rubber) of reinforcing or non-reinforcing
fillers of mineral or carbon black type. Other additives can also be present in these
elastomeric compositions in lesser amounts, such as, sulfur, zinc oxide, magnesium
oxide, cure activators, accelerators, antioxidants, scorch inhibitors, and the like.
[0007] Optionally, these polynorbornene based elastomeric compositions can also contain
up to about 50 parts per 100 parts of polynorbornene by weight of a second rubber.
For example, an EPDM rubber can be added to improve the ozone and age resistance of
the elastomeric composition. In fact, they can be used in lieu of staining antiozonants.
EPDM rubbers are terpolymers of ethylene, propylene, and a diene monomer, such as,
1,3-butadiene. Some representative examples of EPDM rubbers that can be blended into
the elastomeric compositions of this invention are described in U.S. Patent 3,915,907
and U.S. Patent 3,970,133 which are incorporated by reference herein. Nitrile rubbers
can also be added to the elastomeric compositions of this invention in order to improve
their oil resistance. Carboxylated nitrile rubbers can be added in order to improve
the abrasion resistance of the elastomeric composition.
[0008] It is preferred for the elastomeric compositions of this invention to contain from
80 to 200 parts of the plasticizer and from 20 to 70 parts of an incompatible resin
per 100 parts of polynorbornene rubber by weight. If it is desired to blend an EPDM
into such an elastomeric composition then generally from 20 to 40 parts will be employed
per 100 parts of polynorbornene rubber by weight.
[0009] The polynorbornene rubbers useful in this invention are readily available commercially.
For example, CDF Chimie sells polynorbornene under the tradename Norsorex™.
[0010] The plasticizers used in the practice of this invention are composed of light oils
and an aromatic resin. It is important for these oils to be nontoxic to humans since
they may come in contact with human skin. These oils are aromatic, naphthenic, and
cyclic petroleum distillates which have been refined to eliminate polynuclear aromatic
compounds and polar heterocyclic compounds. Hydrogenation can be employed in the refining
process in order to remove certain of these undesired substances. The most important
feature of these oils is their low polar content. Polars (polar heterocyclic compounds)
are generally heterocyclic aromatics containing nitrogen, sulfur, or oxygen. Polar
content can be measured by ASTM Method D2007. The oils employed in the plasticizers
of this invention generally contain less than about 4 weight percent polar compounds
(they have a polar content of less than about 4 weight percent). It is preferred for
the oils used in the plasticizers of this invention to contain no more than 2 weight
percent polar compounds. It is normally most preferred for these oils to have a polar
content of 1 percent by weight or less. These oils also generally contain less than
about 0.25 weight percent sulfur as determined by ASTM Method D2622. It is preferred
for them to contain 0.1 weight percent sulfur or less.
[0011] The oils employed in the plasticizers of this invention have a high aromatic content
and a low content of polynuclear aromatic compounds. These oils generally contain
from about 20 to about 50 weight percent aromatic compounds as determined by ASTM
Method D2007 and preferably contain from 30 to 45 weight percent aromatic compounds.
Polynuclear aromatic compounds have a strong tendency to absorb ultraviolet light
at a wave length of 260 µ (milli-microns). Thus, as an oils content of polynuclear
aromatics increases its absorptivity at 260µ will also increase. The oils used in
the plasticizers of this invention will generally have an absorptivity at 260pas determined
by ASTM Method D2008 of about 8.0 or less, with absorptivities of less than 6.0 being
preferred, and with absorptivities of 4.0 or less being most preferred.
[0012] The oils used in the plasticizers of this invention have molecular weights as determined
by ASTM Method D2502 which normally range from about 200 to about 600 with molecular
weights ranging from about 300 to about 450 being preferred. These oils generally
have viscosities at 210°F (99°C) as determined by ASTM Method D2161 which range from
about 20 to about 200 SUS (Saybolt Universal Seconds). Oil that has viscosities ranging
from about 40 to about 125 SUS are normally preferred.
[0013] The aromatic resins employed in the plasticizers of this invention are prepared by
the polymerization of aromatic monomers. These aromatic resins generally have a relatively
low molecular weight ranging from about 200 to about 800. It is normally preferred
for such aromatic resins to have a molecular weight of 300 to 500 with molecular weights
of about 400 being most preferred. These aromatic resins can contain numerous aromatic
rings, such as, benzene rings, naphthalene rings, anthracene rings, and the like.
[0014] Aromatic resins that are useful in the plasticizers of this invention can be synthesized
by the polymerization of vinyl-substituted aromatic compounds. These vinyl-substituted
aromatic compounds normally contain from 8 to 16 carbon atoms. Some representative
examples of vinyl-substituted aromatic compounds include: styrene, alpha-methylstyrene,
vinyl toluene, 3-methylstyrene, 4-methylstyrene, 4-cyclohexylstyrene, para-chlorostyrene,
3-vinyl-alpha-methylstyrene, 4-vinyl-alpha-methylstyrene, 1-vinyl naphthalene, 2-vinylnaphthalene,
and the like. Terpenes can also be polymerized into aromatic resins that are useful
as plasticizier components for use in the practice of this invention. For example,
alpha-pinene can be polymerized into a polyterpene resin which has excellent properties
for use as the aromatic resin in the plasticizers of this invention. Such polymerizations
are normally initiated by employing free radical generators. Some free generators
that can be used for this purpose include various peroxides, radiation, and ultraviolet
light. The free radical generators that are most commonly used are benzoyl peroxide,
dicumyl peroxide, t-butyl peroxybenzoate, and azodiisobutyronitrile.
[0015] Aromatic resins that can be employed in the plasticizers of this invention are readily
available commercially. For example, Kenrich Petrochemicals, Inc. sells such an aromatic
resin under the tradename
TK Kenflex A. The Arizona Chemical Company also sells an aromatic resin which is a polyterpene
under tradename TM TM Zonarex Alpha 25. Zonarez Alpha 25 has a molecular weight of
about 400, a viscosity at 50°C of 4300 cP (centipoise), and a ring and ball softening
point of 25°C. Hercules Incorporated sells a polystyrene that can be employed as the
aromatic resin in the plasticizers of this invention. It is sold under the TM tradename
Piccolastic A and has a molecular weight that is in the range of 300 to 400.
[0016] The amount of oil and aromatic resin that can be incorporated in the plasticiziers
of this invention can be varied over a wide range. However, it is generally preferred
for the plasticizer to contain from about 5 to about 140 parts of aromatic resin per
100 parts of oil by weight. It is generally preferred for such plasticizers to contain
from 20 to 80 parts of aromatic resin per 100 parts by weight of the oil. These plasticizers
normally have a viscosity at 38°C (100°F) within the range of 4,000 to 10,000 SUS
and have a pour point of 16°C (60°F) to 38°C (100°F). It is more preferable for them
to have a pour point in the range of 24°C (75°F) to 35°C (95°F).
[0017] There are numerous resins which are incompatible with polynorbornene that can be
employed in the rubber compositions of this invention. It should be noted that miscibility
and compatibility mean essentially the same thing. The former refers generally to
liquid systems, whereas the latter usually designates solid systems. In polymer-polymer
systems which are compatible interdiffusion of the polymers results in the polymers
being mixed homogeneously down to a molecular level, whereas in incompatible systems
the minor component only breaks down into small domains. In most cases there is a
lack of interfacial adhesion between such small domains of one polymer and the second
polymer in incompatible systems (blends of incapatible polymers).
[0018] The compatibility of a given rubber-polymer blend can be determined by comparing
the solubility parameters (8) of the rubber and the polymer. As a rule of thumb if
the difference between the solubility parameters of the rubber and polymer is equal
to or greater than one then the rubber and polymer are incompatible. Thus, resins
that have solubility parameters that differ from the solubility parameter of polynorbornene
rubber by one or more are generally incompatible with the polynorbornene. A more detailed
description of solubility parameters and the solubility parameter of numerous polymers
is given by H. Burrell and B. Immergut, "Solubility Parameter Values,"IV-341 in Polymer
Handbook, J. Brandrup and H. Immergut, eds., Wiley-Interscience, New York, 1966, which
is incorporated herein by reference. One incompatible resin that can be used with
great success in the rubber compositions of this invention is sold under the tradename
Staybelite™ and is a hydrogenated pine tar resin, having a saponification number of
about 170 and a melting point of about 65°C to 75°C.
[0019] After the elastomeric compositions of this invention have been molded into the desired
shape they are cured at an elevated temperature generally in excess of 150°C. Normally
sulfur is added to the elastomeric compositions of this invention as a curative in
ordinary amounts ranging from about 0.5 to 5 phr with the amount of sulfur present
preferably being in the range of 2 to 3 phr. It is advantageous to use cure activators,
such as, zinc oxide, at a level of about 2 to 6 phr and preferably at a level of 4
to 5 phr. It is also advantageous to employ stearic acid at a level of 0.5 to 4 phr
and preferably at a level of 1 to 2 phr in such elastomeric compositions. Well-known
antioxidants and antiozonants can also be employed in cure recipes in normal levels
generally ranging from 0.5 to 2 phr. A processing aid is also normally employed in
elastomeric compositions in an amount ranging from 5 to 50 phr with 20 to 30 phr being
preferred. Vulcanized vegetable oil is a preferred processing aid for use in such
elastomeric compositions.
[0020] This invention is illustrated by the following examples which are merely for the
purpose of illustration and are not to be regarded as limiting the scope of the invention
or manner in which it may be practiced. Unless specifically indicated otherwise, parts
and percentages are given by weight.
Example 1
[0021] This example has been included in order to illustrate the usefulness of the elastomeric
compositions of this invention as a structural material from which innersoles for
shoes can be made. 100 parts TK of polynorbornene rubber (Norsorex ), 135 parts of
TK Kenflex L (plasticizer), 25 parts of a vulcanized
Tx vegetable oil, and 40 parts of Staybelite (an incompatible resins) were mixed on
a mill with about 5 parts of zinc oxide, 2 parts of stearic acid, 2.3 parts of sulfur,
4 parts of a sulfenamide, 0.8 parts of a methyl thiuram accelerator, and 1 part of
an antioxidant to produce an elastomeric composition. This elastomeric composition
was compression molded and cured at a temperature of a bout 150°C for 30 minutes to
produce an innersole for a shoe. This innersole exhibited excellent shock absorbing
properties. This innersole compared very favorably to polyurethane innersoles commercially
available in running and walking tests. The shoe manufactured using this innersole
exhibited shock absorbing properties that were very beneficial to the wearer as well
as being very compatible.
Example 2
[0022] The elastomeric compositions of this invention have an excellent ability to dissipate
energy. In other words, they provide a high degree of damping.
[0023] In this example, a Rheovibron (Model DDV-II), direct reading dynamic viscoelastometer,
manufactured by Toyo Measuring Instruments Co., Ltd. was employed to determine the
ability of the elastomeric composition specified in Example 1 to dissipate energy
at various temperatures. The Rheovibron provides a direct reading of Tan δ which is
essentially the materials damping ability or ability to absorb energy.
[0024] The elastomeric composition employed in the shoe innersole in Example 1 was tested
on the Rheovibron over a temperature range from -10°C to 30°C at a heating rate of
3°C per minute at frequencies of 3.5 Hz, 11 Hz, and 110 Hz. The approximate Tang values
of the elastomeric composition at various temperatures is given in Table I as determined
at 3.5 Hz, 11 Hz, and 110 Hz.

[0025] As can be determined from Table I the elastomeric composition of this invention has
high Tan δ values over a useful temperature range. In other words, it has an ability
to dissipate large amounts of energy over the temperature range wherein shoes are
normally used. Thus, it has a great capacity to absorb shock and acts as good protection
for the foot in walking and especially in running.
Example 3
[0026] This example is included in order to show that the elastomeric compositions of this
invention compare very favorably in their ability to absorb energy with TK Sorbothane
which is a polyurethane commonly used as a material for making innersoles for shoes.
The procedure used in Example 2 was repeated in this example except that Sorbothane
was used in place of the elastomeric composition of this invention tested in Example
2. Tans values are given in Table II.

[0027] As can be determined by comparing Table I with Table II the elastomeric compositions
of this invention have a higher capacity to absorb energy at a
™ temperature of 10°C to 20°C than does Sorbothane . This is every important since
shoes are most commonly used in this temperature range.
[0028] While certain representative embodiments and details have been shown for the purpose
of illustrating the invention, it will be apparent to those skilled in this art that
various changes and modifications may be made therein without departing from the scope
of the invention.
1. An elastomeric composition which is characterized by containing: (1) a polynorbornene
rubber, (2) a plasticizer, and (3) a resin which is incompatible with the polynorbornene
rubber.
2. A composition as specified in claim 1 which is characterized by containing 20 to
400 parts of said plasticizer and 5 to 120 parts of said resin which is incompatible
with the polynorbornene per 100 parts of said polynorbornene rubber.
3. A composition as specified in claim 2 which is characterized by containing 80 to
200 parts of said plasticizer and 20 to 70 parts of said resin which is incompatible
with the polynorbornene per 100 parts of said polynorbornene rubber.
4. A composition as specified in claim 2 or 3 characterized in that said plasticizer
is comprised of
(a) an oil having a polar content of less than about 4 weight percent and an absorptivity
at 26%as determined by ASTM Method D2008 of about 8 or less and
(b) an aromatic resin; wherein said plasticizer contains about 20 to about 80 parts
of said aromatic resin per 100 parts of said oil by weight.
5. A composition as specified in claim 4 characterized in that said oil contains from
about 20 to about 50 weight percent aromatic compounds.
6. A composition as specified in claim 4 or 5 characterized in that said oil contains
no more than about 2 weight percent polar compounds and has a molecular weight ranging
from about 200 to about 600.
7. A composition as specified in claim 4, 5 or 6 characterized in that said aromatic
resin is a polymer of a vinyl-substituted aromatic compound containing from 8 to 16
carbon atoms wherein said aromatic resin has a molecular weight of about 200 to about
800.
8. A composition as specified in claim 4, 5 or 6 characterized in that said aromatic
resin is a polyterpene having a molecular weight ranging from about 200 to about 800.
9. A composition as specified in claim 4, 5 or 6 characterized in that said aromatic
resin is polystyrene.
10. A composition as specified in any of the preceding claims characterized in that
said resin which is incompatible with polynorbornene is a hydrogenated pine tar resin.